This invention relates to an electric field emission cold cathode having a current limiting element connected to an emitter, and further relates to a display device using the electric field emission cold cathode as an electron gun.
An electric field emission cold cathode is an element for concentrating the high electric field at the tip of a sharp-pointed conical emitter using the emitter and a gate electrode having an aperture of size on the order of submicrons and formed adjacent to the emitter, so as to emit electrons from the tip of the emitter under vacuum. In the electric field emission cold cathode, since the emitter and the gate electrode are located quite close to each other, the large flow of current may occur in the emitter due to discharge induced by gas or the like during operation so that a material of the emitter is melted to cause a short circuit between the emitter and the gate electrode. As a countermeasure for this, there was developed an element with a resistance layer which was formed in series with an emitter for limiting the current upon discharge so as to prevent the melt damage of the emitter. However, in this method, there was a problem of increase in operating voltage caused by the potential drop across the resistance layer even during a normal operation other than upon discharge.
Under these circumstances, there has been proposed a method, wherein an active element having a saturation current characteristic is formed at an emitter for controlling the current flowing in the emitter. Conventional electric field emission cold cathodes of this type have structures as shown in FIGS. 1 and 2, which are disclosed in, for example, Japanese Unexamined Patent Publications (A) Nos. 130,281 of 1995 and 249,026 of 1992.
The first conventional electric field emission cold cathode shown in FIG. 1 will be first explained. FIG. 1 is a sectional view of the first conventional electric field emission cold cathode. In FIG. 1, the electric field emission cold cathode includes sharp-pointed conical emitters 7a (only one is shown) made of molybdenum (Mo), a gate electrode 4b made of tungsten (W) and surrounding the emitters 7a, an insulation film 3 in the form of an oxide film, an n-type silicon 17 connected to the emitters 7a, a p-type silicon 16 surrounding the n-type silicon 17, and a p-type leading electrode 4c made of W and connected to the p-type silicon 16, which are formed on an n-type silicon substrate 11 connected to a substrate electrode 8 working as a cathode electrode. In this prior art, the n-type silicon 17, the p-type silicon 16 and the n-type silicon substrate 11 form a junction field effect transistor and, by changing the voltage applied to the p-type silicon 16, the current flowing in the n-type silicon 17 can be controlled. Further, for ensuring voltage-withstanding, the concentration and depth of the n-type silicon 17 between the emitter 7a and the n-type silicon substrate 11 are set so as not to be applied with a electric field intensity greater than the breakdown electric field intensity of silicon.
Now, the second conventional electric field emission cold cathode shown in FIG. 2 will be explained. FIG. 2 is a sectional view of the second conventional electric field emission cold cathode. In FIG. 2, the electric field emission cold cathode includes sharp-pointed conical emitters 7a (only one is shown) made of molybdenum (Mo), a gate electrode 4b made of W and surrounding the emitters 7a, a cathode electrode 4a, an insulation film 3 in the form of an oxide film, an n.sup.+ -type silicon 18, an n-type silicon 17 and an insulated gate field effect transistor (IGFET) gate 19, which are formed on a p-type silicon substrate 1. In this prior art, the n-type silicon 17, the n.sup.+ -type silicon 18, the p-type silicon substrate 1, the cathode electrode 4a corresponding to a source electrode and the IGFET gate 19 form an IGFET and, by changing the voltage applied to the IGFET gate 19, the current value can be controlled. Further, for setting a withstand voltage of the IGFET to be not less than a voltage between the gate electrode 4b and the emitter 7a (cathode electrode 4a) upon electron emission, the n-type silicon 17 is used as a pinch-off resistor for suppressing voltage increase of the n.sup.+ -type silicon 18 connected to the emitter 7a so as to ensure the voltage-withstanding.
In the foregoing conventional method wherein the current value is controlled by the active element, the first drawback is that, for controlling the current value, the element is enlarged, peripheral circuits of a device using the element are increased and thus the whole device structure becomes complicated. Specifically, the additional electrode and a power supply connected thereto are required for controlling the current value other than the cathode electrode, the gate electrode (and an anode electrode for receiving emitted electrons) and independent power supplies connected thereto, which are necessary for the normal electric field emission cold cathode. The first conventional electric field emission cold cathode requires the p-type leading electrode 4c for controlling the voltage applied to the p-type silicon 16, and the second conventional electric field emission cold cathode requires the IGFET gate 19. Particularly, in the second conventional electric field emission cold cathode, the element requires a gate oxide thin film under the IGFET gate 19 and further requires the IGFET gate 19 separately from the gate electrode 4b and the cathode electrode 4a so that the element structure becomes complicated.
The second drawback is that, for ensuring the voltage-withstanding when the current flows in a depth direction from the emitter 7a to the substrate electrode 8 corresponding to a cathode electrode in the first conventional electric field emission cold cathode, it is necessary to form the n-type silicon 17, whose current is controlled by the p-type silicon 16, so as to have a depth of not less than 10 .mu.m over a constant width. However, this is difficult in view of fabrication. Specifically, when forming the n-type silicon using the diffusion method, since expansion occurs in a width direction as advancing deeper, it is difficult to achieve the constant width. Even when the ion implantation method is used, since expansion in a width direction differs in a depth direction, it is necessary to carry out the implantation in a plurality of times and further, implantation masks become thicker so that the fabrication processes become complicated and take time.
The third drawback is that, when using the n-type silicon 17 as the pinch-off resistor for ensuring the voltage-withstanding in the second conventional electric field emission cold cathode, the resistance value (current value) is liable to change depending on the voltage applied to the gate electrode 4b so that the operation becomes unstable. This is caused by the fact that, since the gate electrode 4b with the positive potential higher than the emitter 7a is provided over the n-type silicon 17, an accumulation layer of n-type electrons is formed at the side of the n-type silicon 17 adjacent to the insulation film 3 to reduce the pinch-off resistance of the n-type silicon 17 so that the current value to be controlled changes depending on the gate voltage.
The fourth drawback is that, although the withstand voltage of the element using the conventional active element is set to be not less than the voltage applied between the emitter 7a and the gate electrode 4b upon electric field emission, since there is no limit to the flow of current per emitter, the damage upon discharge can not be prevented. The reason for this is that, according to the researches made by the present inventor, if the current not less than 10 mA flows per emitter of Mo on the silicon substrate upon an occurrence of discharge between the emitter and the gate electrode, the emitter is melted to cause a short circuit relative to the gate electrode. Accordingly, in the foregoing prior art, even if the elements are partly damaged, the remaining elements may be free of damage and normally operated, however, it is difficult to prevent an occurrence of short-circuited damage itself.